The invention is based on an amplifier circuit with filters for digital and analog signals, according to the precharacterizing clause of claim 1. Multistandard IF amplifiers (IF=intermediate frequency) are currently used for analog reception for various standards. The IF amplifier must select appropriate signals for each standard. Selection takes place in the IF range at a respectively fixed frequency. Filters are used for the selection. For analog reception, a Nyquist slope is required in order to obtain a suitable signal. This is not necessary for a digital signal.
Currently, if an analog and a digital signal are processed in a signal processing stage, then the corresponding analog or digital signal path is connected as a function of the received signal. A disadvantage with this is the relatively high outlay for analog signal processing.
It is an object of the invention to improve the selectivity of a filter in the path of the analog signal without the need of additional circuit means. This object is achieved by the features of the invention which are specified in claim 1. Advantageous developments of the invention are specified in the subclaims.
According to the invention, filters for digital and analog signals are employed. For analog signal reception, the filter for digital signal reception is connected into the path of the analog signal. By connecting the filter for the digital path upstream of the filter for the analog path, it is possible to do without selection means, for the analog signal, otherwise required in addition to the main selection. For digital signal reception, the filter of the analog path is not connected, since the digital signal requires the greatest possible bandwidth, whereas the filter of the analog path would limit the bandwidth. In order to decouple the filters for analog and digital signal processing, a decoupling stage is employed. The decoupling stage is preferably designed as an emitter follower. The filters for analog and digital signal processing are formed by surface acoustic ware filters because of their low costs. But also other types of filters can be employed like LC-filters. The filters are preferably arranged in a cascade circuit.
An advantage with an arrangement of this type is that the additional selection means used up to now in the analog signal processing path can be obviated. This additional selection is then carried out by the filter within the path of the digital signal.
A further advantage is that, for the surface acoustic wave filters, characteristics relating to amplitude frequency response and group delay time can be configured independently of one another. By virtue of the cascade circuit, a desired group delay time for the analog signal output can be configured more flexibly. This is possible by virtue of the superposition of the group delay times of the two filters. Ripples in the signal profile at the output of the surface acoustic wave filter in the path of the analog signal, which are otherwise unavoidable, can be compensated by the cascade circuit, by its signal profile being of opposite phase to the surface acoustic wave filter within the path of the digital signal. The desired group delay time profile of the surface acoustic wave filter in the analog path can be obtained by the appropriate profile of the surface acoustic wave filter in the digital path.